The invention concerns a method for coating of a metallic coil or of metallic sheets with a composition for treatment or pre-treatment whereby the such treated metallic material is further on shaped to an article like a container or a casing, especially to a can, and then cleaned and optionally further either chemically pretreated and then coated with ink or paint or chemically treated. In the following, the production line of a two-pieces aluminum can is selected to demonstrate on the one side the conventional process of today and on the other side a process according to the invention.
In today can production, an aluminum can plant buys aluminum coils at an aluminum coil mill having an aluminum cold rolling facility. The aluminum coil stock is typically of a specific alloy type which is used in many can plants. These aluminum coils are then sent to the can plant having a so called post-lube applied on the surface. The post-lube is an oil or an ester based composition, typically having a considerable amount of vegetable oil or mineral oil or both. The post-lube aids in the corrosion protection of the metallic material.
The aluminum alloy coil used for the can production is often rolled down to a wall thickness in the range from 0.45 to 0.25 mm at the aluminum mill, whereas a wall thickness e.g. of 0.25 mm is reduced during the shaping process at the can plant to a wall thickness e.g. of 0.10 mm, often in about 4 or 5 process steps in a body-maker.
First, at the front end of the can plant, the coil, which carries typically an oil containing post-lube upon its surfaces, is hold in an uncoiler for unwrapping the coil.
Then, a lubricant composition is applied which may contain oil, ester(s), emulsifier(s) or water or any combination thereof upon the coil e.g. with the aid of a spray nozzle. It may be called “post-lube” too and may be of the same or of a similar composition compared with the first post-lube. This lubricant composition is applied to the coil, which is then used for aiding in the shaping of the can, typically just before or in the “cup-maker” or both. After the cup-maker has produced pre-formed cans called “cups”, the cups are transported to a so called body-maker machine (“body-maker”).
The body-maker typically uses a composition which contains oil, emulsifier(s), ester(s), coolant(s) or any combination thereof for the further shaping and the cooling of the tools and the shaped component. This equipment shapes the cups by a drawing and wall ironing process to the final shape and to the final surface quality of the surfaces as it is well-known e.g. as a beer can or as a cola can. The drawing and the wall ironing process or similar shaping processes cause so much force onto the aluminum material that the aluminum alloy in the tools flows like in a cold-forming operation. After the shape of the so-called “body” is generated, the top of the drawn cup is cut (“trimmed” in a “trimmer”), and the cans are transported to the so-called “washer” having several baths where in today processes, in different process steps cleaning is performed and where typically different chemicals are applied in different baths. In between and optionally at the end of the washer too, there is at least one water rinsing.
Aluminum cans are today produced at a speed of 1000 to 4000 can units per minute in one line, which are often drawn and wall ironed by up to 10 parallel body-makers, but often only drawn to cups by only 1 cup-maker before in this line.
The typical (pre-) treatment process in a can washer may often comprise the following stages:
1. Pre-rinsing—stage 0
2. Pre-cleaning—stage 1
3. Acidic cleaning—stage 2
4. Rinsing A/B—stage 3a
5. Dome stain (pre-) treatment—stage 4
6. Rinsing A/B—stage 5
7. DI rinsing—stage 6 (deionized, often even recycled, water)
8. Mobility Enhancer—stage 7.
The can bodies coming from the body-maker typically have very smooth outer surfaces, but need to be cleaned. Gardobond® S 5240 und Gardobond® 45 CR of Chemetall GmbH may be used in the (pre-) cleaning stages to get rid of oil, dirt and other contaminants like the burnt oil and other burnt organic components which may cause the can body to look black and to remove thereby the content of post-lube, of cupping lube and of body-maker coolant/lube. Such aqueous acidic cleaning compositions may contain free fluoride or Fe2+ together with at least one oxidizing agent like a peroxide. But the longer or the stronger the etching in the acidic bath is, the rougher the can body may become. The color of the can body may even turn to white, if there is a too strong etching. And the can body has to be rejected too, if it has a very high friction. The can bodies cannot be transported in an adequate way without application of a mobility enhancer if they show a certain roughness. By lowering the etching rate, there is less or no need for applying a mobility enhancer.
The can may then be (pre-) treated with an aqueous composition for a conversion coating typically based on Zr, F and PO4, e.g. with the product Gardobond® 1450 N or Gardobond® 764 of Chemetall GmbH or with Alsurf 450® of Nippon Paint Corp. in the so-called “stage 4 process” or “dome stain treatment” of the washer so that the bottom (dome) of the can is protected during the pasteurization against corrosion as the pasteurization is often necessary especially for beer cans. This dome stain treatment typically leads to a zirconium containing coating having a zirconium content to be measured as elemental zirconium in the range from 2 to 14 mg/m2 Zr. The application of such compositions in a can washer is a difficult process due to the limited stability of the system and due to the sludge generation. The generated coating often affects the mobility of the cans. The mobility of the cans which stand and roll one parallel to the other standing on a transportation belt or on a transportation mat is significantly influenced by the gliding properties of the can surfaces and of the coatings on the can bodies. The mobility is directly related to production speed in the can plant. The higher the mobility is, the higher may be the production speed and the production capacity.
By applying a so-called “mobility enhancer” to the can body especially in stage 7 of the washer, e.g. an aqueous composition on the base of a mixture of surfactants in aqueous solution, the gliding ability of the mostly rough surface of the can body is improved.
The cans may be shipped to a brewery, where e.g. beer may be pasteurized either prior to filling it into the cans or after having filled it into the cans. In the last case, especially the not further treated outer surface of the dome may underlie corrosion e.g. by blackening if there is an insufficient corrosion protection. The pasteurizing is often conducted with hot water of about 75 to 95° C. At this temperature, the dome would become white to grayish and sometimes even black because of the start of corrosion at the metallic surface if it is not corrosion protected. Therefore, a protection of the dome outside surface is important as only the other outer surfaces as well as to the inner surfaces independent one from the other are painted or printed with ink or paint or both. Such a color change has to be avoided.
We have found that the content of phosphoric acid of a typical cola may corrode the wall of a typical aluminum can in about 6 hours if there is no inside corrosion protection. Therefore, even breakings and cracks of the metallic material and of its coatings should be reduced or even avoided to minimize the risk of corroding such cans not only on the inner surface, but even to avoid crevice corrosion.
This conventional process in a can washer often shows the following disadvantages:
The succession of baths and (pre-) treatments of the can bodies in the washer is complex and difficult, and it is a sensitive system, even in relation to the shaping operations before. The most disadvantageous effects are related to the dome stain (pre-) treatment and to the mobility enhancer (pre-) treatment.
1) The dome stain (pre-) treatment is often disadvantageous because of:
a) The effect of reducing the glidability of the can bodies because of the perhaps more or less crystalline and typically relatively rough coating generated with the dome stain composition.
b) The loss of paint adhesion in the necking area of the can bodies, which is nearby to the area where the lid will be joined to, as the more or less crystalline dome stain coating is not flexible enough to be significantly bent in the necking area and causes micro-cracks and fractures during bending which causes micro-cracks and fractures of the paint layer applied upon the dome stain coating too whereby the micro-cracks and fractures occur primarily in the segments of convexly bent outer regions, especially if they are coated with a highly pigmented ink or highly pigmented paint or both, whereby white bare rust may later occur; therefore, it would be a great advantage to avoid this failure type.
c) The temperature of the dome stain (pre-) treatment bath is often in the range from 35 to 60° C. which is expensive.
d) The costs of the chemicals in the dome stain (pre-) treatment.
e) Sludge generation, which causes pauses for cleaning the baths during which there is no production in the line.
f) The disposal of waste water, chemicals and sludge.
g) In the bath for a dome stain (pre-) treatment only a very low sulfur content is acceptable, but easily a certain sulfur content of the acidic cleaning bath may be introduced: If a body is standing upwards and not downwards, which occurs in some situations, such upstanding can body in stage 4 introduces sulfuric acid and other acids from the acidic cleaning solution into the bath of stage 4, which should therefore have a continuous overflow and a loss of chemicals to ensure a very low sulfur content in the bath.
h) The (pre-) treatment time to be used is only very few seconds for one can body, but if the can transportation speed is reduced or if there occurs a line stop, the dome stain coating has more time to develop and is therefore thicker and rougher. Then the glidability of this coating is significantly reduced.
Therefore, it would be a significant advantage to avoid a dome stain (pre-) treatment or to use a dome stain (pre-) treatment which does not generate a rough crystalline coating like coatings on the base of at least one phosphonate as it is possible to use so-called “self-assembling molecules” (SAM) on the base of at least one compound selected from the group of phosphonic acids, phosphonates and their derivatives and/or to use a dome stain (pre-) treatment with less environmentally unfriendly consequences.
A mobility enhancer shall create a well glidable coating on the surface of the can body, so that a more or less rough surface is flattened and made better glidable than without such coating.
2) The use of a mobility enhancer is often disadvantageous because of:
a) The mobility enhancer composition—in the following called “mobility enhancer”—is today often an aqueous composition on the base of surfactants or esters or both. The higher the concentration of the mobility enhancer is or the longer it is applied e.g. during a line stop, problems may occur in painting or printing the can afterwards: The more hydrophilic the surface coated with the mobility enhancer is, the easier may occur wetting problems, if an ink or a paint is used which is more hydrophobic as the typically used paints or inks or both for the outer surfaces of a can or an article are more hydrophobic. There may then a problem occur because of insufficient adherence to the surface. But typically, there does not occur a problem on the inner surfaces of a can or of an article, as there is often used a hydrophilic ink or paint or both.
b) There may occur a dirt from a mobility enhancer which may cause a type of failure called “salt rings” which may be caused by a too high concentration of a mobility enhancer bath, especially occurring when a high mobility enhancer concentration is applied to the standing can body, when the mobility enhancer forms a liquid film ring at the bottom and dries on. Such salt rings are a reason for rejection of the such coated shaped bodies.
The percentage of rejections because of the dome stain (pre-) treatment and of the mobility enhancer (pre-) treatment may be at least 0.1% of the whole can production, perhaps even sometimes more than 1%, which is a high cost factor in such a mass production. These two production stages seem to be typically the stages with the highest failure rates. One can production line only may have costs because of the rejection of cans in the range of vaguely half a million C= per year.
It is therefore an object of the invention to propose an easier or cheaper method for producing hollow articles like cans and casings. It is another object of the invention to propose a method for producing hollow articles like cans and casings in a less complex, less instable or shorter process succession.
We have now found that there may often occur micro-cracks in the aluminum alloy of cans at the dome outside surface, which seem to arise from the shaping in the body-maker. Such cracks may hold oil inside, as the capillary forces are very strong, even despite heating and high spray pressures. The oil may remain in the micro-cracks, so that the oil may spread out of the micro-cracks if the can is heated as the inside of the can is not yet painted. The later-on (subsequently) applied water-based paint is then not able to cover the small oil covered areas of the inside surface. Then there is no paint in such areas, and at these flaws, there is no corrosion protection. Therefore, it is preferred to optimize the shaping process even so to reduce the numbers and the size of the micro-cracks during the shaping steps.
We have now found that there are several advantages if the shaped can body is not coated with the specific chemicals of the “stage 4 process” conventionally used today on the base of Zr, F and PO4 in stage 4 of the washer, but if the metallic coil or the metallic sheets are already coated before.
We have now found that at least a part of the content of zirconium applied in a zirconium rich coating on coil may remain on the surface or in the surface layer or both of the metallic material during the shaping and even during the cleaning after the shaping, which is very surprising.
We have now found that a can may be produced with a perfect dome stain resistance without using the conventional “stage 7 process” with a mobility enhancer, if a metallic coil or if metallic sheets are precoated with an adequate corrosion resistant coating. This stage may be therefore omitted or may be replaced e.g. by a rinsing stage with water or with water having a low surfactant(s)' content. Such an omission is only possible if the metallic material stock had shown an adequate coating before the shaping which remains during the process at least partially on the metallic surface or leads to a modified metallic surface or both.
An investigation revealed that zirconium is present at the surface of a can body, although no dome stain (pre-) treatment or no other zirconium containing composition had been applied in the washer.
It was surprising that the zirconium content of the zirconium containing passivation layer present on the metallic coil or on the metallic sheets tested was not totally removed in the shaping and in the thereon following cleaning process. Therefore, it is believed that the zirconium content of this coating was transformed into the surface of the aluminum alloy during the shaping especially during the drawing and wall ironing steps in the body-makers, especially due to the high pressure and perhaps due to the high temperatures present during shaping.
We have found that the coating applied on the metallic surface is able to aid in the shaping process of the metallic coil or metallic sheets as well as in the further shaping of the pre-shaped bodies like cups and (can) bodies, especially in the cup-maker or in the body-maker or both of a can manufacturer.